Fuel systems typically employ multiple fuel injectors to deliver injections of high pressure fuel into an engine for combustion. Each fuel injector typically includes a nozzle assembly having a pressurized chamber configured to contain a volume of pressurized fuel. Each injector further includes a needle valve element that is slidably disposed within the pressurized chamber. In response to a deliberate injection requirement, the needle valve element moves to allow the pressurized fuel to flow from the pressurized chamber into a combustion chamber of the engine.
Fuel injectors operate at injection pressures of up to 200 MPa in order to obtain a fine fuel spray for rapid mixing and reduced emissions. Such pressurized fluid systems, however, are susceptible to the phenomenon known as “cavitation.” Cavitation generally refers to the formation of vapor bubbles within a fluid stream when, for example, the fluid's operational pressure drops below the fluid's vapor pressure. As applied to a fuel system, cavitation occurs when the speed or velocity of the flowing fuel increases such that the pressure in the system drops below the vapor pressure of the fuel, resulting in local vaporization of the fuel. The local vaporization, in turn, creates a cavity (i.e., hole) or void within the flowing fuel. This low-pressure cavity generally comprises a swirling mass of fuel droplets and vapor bubbles.
Once formed, the low-pressure cavity is generally swept downstream into a region of high pressure, such as, for example, an eddy zone, where it suddenly collapses as the surrounding fluid rushes in to fill the void. As the cavity collapses, each and every vapor bubble within the cavity implodes, releasing a momentary burst of concentrated energy. In instances where a cavity's point of collapse is in contact with a boundary wall, such as, for example, the material surface of the fuel injector nozzle, the concentrated energy released from the bubble implosion locally stresses the wall surface beyond its elastic limit and causes erosion of wall material.
Cavitation can be extremely problematic to the performance of fuel injectors. Specifically, cavitation can affect the mass flow and spray quality characteristics of the fuel injector. Moreover, cavitation can lead to structural damage and a shortened life of the injector. It is, therefore, desirable to experimentally study cavitation structures inside of a fuel injector, and more particularly a fuel injector nozzle, to better understand this economically important flow.
One method of visualizing cavitation in a diesel fuel injector is discussed by Haiyun Li in an article titled “Visualization of Cavitation in High-Pressure Diesel Fuel Injector Orifices,” in Atomization and Sprays, 2006, 875:886 (“the Li article”). The Li article discloses an apparatus for conducting true-scale, true-pressure visualization of cavitating liquid fuel in a modern diesel fuel injector. Specifically, the Li article discloses an experimental rig consisting of a gaseous nitrogen driver system, a high-pressure flow system, a test section, and a downstream reservoir. The high-pressure flow system includes an intensifier for pressurizing the liquid fuel up to 220 MPa. The intensifier is connected, via a hose, to an optically accessible orifice mount. The orifice mount contains a single acrylic test orifice. The test orifice has a xenon arc lamp on one side as an illumination source, and a camera on the opposing side. In operation, the intensifier pressurizes fuel at a set injection pressure. The fuel then moves from the intensifier to test orifice, where images of both the internal flow and the spray are acquired.
Although the Li article discloses an apparatus for visualizing cavitation at an orifice hole of a fuel injector, the apparatus may not be applicable for studying cavitation inside of a fuel injector nozzle as the apparatus does not include a model of, for example, the nozzle sac of the fuel injector. Furthermore, the apparatus described in the Li article may not have the flexibility to control the duration, frequency, and number of fluid injections into the test section and therefore may not capture cavitation under varying operating conditions.
The present disclosure is directed to improvements in the existing technology.